The GPL is a great idea, but it is the deep end of the pool. The idea of total freedom, and the responsibility to give-back in maintaining that freedom, is an awful big step for corporations to take, steeped as most are in the idea of "intellectual property."
Open source lets companies go just as far down this road as they wish. They can call it "blended source," or "mixed source," they can support a wide variety of BSD licenses, they are free to experiment, to set their own strategies.
What most discover, over time, is that they get only as much as they give. Blended source companies get less from their communities than BSD companies, which in turn get less help than those using the GPL.
Ad big companies seek to reduce their expenses for maintaining and growing software, they step out futher along the continuum. This is often done through donating code, or supporting projects, rather than through changing license terms. As they see contributions come back to them they become increasingly generous.
Getting to the GPL, in other words, is a long journey. But thanks to the term open source, there are now 1,000 steps along the way. And without it, most firms would not take that first step.